Generally, wireless communication systems operate in a dedicated or licensed radio frequency (RF) spectrum. In other words, base stations and wireless terminals are communicated with one other through the dedicated spectrum (or licensed spectrum) authorized by telecommunications providers. However, due to the increasing demand for bandwidth from mobile data services, the existing bandwidth in the licensed spectrum is gradually becoming inadequate in use. Therefore, the 3rd Generation Partnership Project (3GPP) introduces Licensed-assisted Access (LAA) and Carrier Aggregation (CA) for allowing wireless communication systems to operate in an unlicensed spectrum for data communications, so as to increase communication bandwidth for mobile data service.
As one of the challenges for wireless communication systems operated in the unlicensed spectrum is the coexistence of a base station of the wireless communication system (in short, a base station) and other devices (e.g., Wi-Fi devices or other base stations) operated in the unlicensed spectrum. The base station is allowed to perform a data communication in the unlicensed spectrum only after completing a Clear Channel Assessment (CCA) procedure. Due to the undetermined condition of the unlicensed spectrum, during the data communication with a user equipment in the unlicensed spectrum, in addition to a boundary of a subframe aligned with a communication starting time, the base station is allowed to start the data communication at the boundary of the second Long Term Evolution (LTE) slot in the subframe.
Considering the computing capability of hardware in the base station, the base station needs to pre-schedule each downlink data communication. With the undetermined condition of the unlicensed spectrum, a completion time of the CCA procedure can be extended or shorten according to the state of the unlicensed spectrum. Consequently, when the completion time of the CCA procedure is earlier or later than said estimated time and the base station still performs the data transmission after the completion time of the CCA procedure, the user equipment may be unable to find Downlink Control Information (DCI) for the user equipment prepared by the base station from a search space in a Physical Downlink Control Channel (PDCCH), resulting in the failure of the downlink data communication.
In light of the above, due to the undetermined condition of the unlicensed spectrum, it is difficult for the base station to accurately estimate a waiting time before the downlink data communication. As a result, the downlink data communication starting at the expected time is unusual. Under the premise of the limited computing capability of hardware in the base station, finding a way to schedule downlink radio resource in the unlicensed spectrum for the improvement of the overall spectrum utilization is a challenge in the design and implementation of the base station.